456 ON USEFUL AND BOOK I. 



This may point out the necessity of attending to the natural 

 soils and situations of trees and shrubs, the qualities of their 

 products, their uses in the arts, and their relative value. 



Where shelter or shade is the principal consideration, the qua- 

 lities of each require to be investigated, and those forms used 

 which are best adapted to that purpose. The object of shelter 

 is to produce heat; and to protect cattle and pasture from the 

 inclemency of the weather. Hence, the trees used for this 

 purpose should be clothed with branches and foliage from the 

 ground upwards. The foliage should be perpetual, and, if 

 possible, so compact upon the spray and branches as not to be 

 easily permeable by the wind, such as is that of the resinous 

 tribe of evergreens. This may be thought too nice a distinc- 

 tion; but any person would be sensible of the difference, if in 

 a windy day he were to stand alternately under the shelter of a 

 group of hollies and spruce firs, of equal magnitude. 



The object of shade is to produce cold ; that men or cattle may 

 enjoy the cool refreshing breeze unmolested by the mid-day sun. 

 For this purpose, it is essentially necessary that the stems of 

 the trees be free from branches to a considerable height, in or- 

 der to promote the free circulation of the air. It is true, most 

 trees may be trained in this form; but the operation would in- 

 jure the character or use of some kinds, whilst others are greatly 

 improved by pruning. The shade of some trees, as the walnut, 



